The invention relates in first instance to a food processor with a mixing vessel and a drive for an agitator in the mixing vessel, the mixing vessel being heatable in its lower region and having on the bottom a dome which has the securing mount for the agitator.
A food processor of this type is known, for example, from DE-A1 44 14 821. The content of this patent application is hereby incorporated in full in the disclosure of the present invention, including for the purpose of incorporating features of this patent application in claims of the present invention. In the case of this known solution, heating up of the mixing vessel and of the food contained therein takes place by means of a resistance heating element which externally encloses the mixing vessel in its lower region and is formed for example as a circular resistance heating wire which is carried by a sleeve clamping the mixing vessel in the mixing-vessel mount.
With regard to the prior art described above, developing a food processor of the type in question in an advantageous way, in particular with regard to improved heat transfer, is seen as a technical problem for the invention to solve.
This problem is solved in that a resistance heating element with printed conductors applied to a dielectric is fitted on the base of the mixing vessel, the generally circular, disk-shaped heating element having a central recess which is adapted to the dome. As a result of this configuration, the mixing vessel and the food contained in it are heated directly. There are smaller losses in comparison with the known prior art. The heat introduced is transferred directly into the food. Furthermore, by dispensing with large masses for heating, the controllability of the heating system is improved and the heating-up rate is increased. What is more, uniform temperature distribution is made possible by disposing the heating element in a specific way. On account of the increased efficiency, the required electrical power is minimized, whereby the prescribed duty cycles are shortened, with an accompanying increase in the control accuracy. It also proves to be advantageous that the way in which the resistance heating element is disposed and configured according to the invention makes the construction less susceptible to tolerances in comparison with conventional systems, in which known systems an exact engagement between the mixing vessel and the heating is required.
The invention also relates to a food processor with a mixing vessel and a drive for an agitator in the mixing vessel, the mixing vessel being heatable in its lower region and having on the bottom a dome which has the securing mount for the agitator. Here, for the advantageous further development of a food processor of the type in question, it is provided that a resistance heating element with printed conductors enveloped by a dielectric is fitted on the base of the mixing vessel. For example, a tubular heating element which is enveloped by a dielectric and is secured on the base of the mixing vessel may be provided here. Here, too, the food is heated directly, with the advantage of a lower loss, since the heat introduced is transferred directly into the food. What is more, the controllability of the heating system is improved and the heating-up rate is increased.
Independently of the configuration according to the invention of the resistance heating element fitted on the base of the mixing vessel, it is further provided that the printed conductors are secured on the outside of the base of the mixing vessel with the dielectric interposed. In this way, a thick-film heating element or a tubular heating element can be disposed directly on the base of the pot-shaped container or separately on a carrier plate which can be associated with the base of the container. With the heating element disposed on a separate carrier plate, the latter can be both securely fastened to the base of the mixing vessel or disposed removably on the base of the vessel. Furthermore, it is also conceivable to locate the heating element on a mixing-vessel base formed on a removable lower part of the mixing vessel. There is also the possibility of a resistance heating element formed as a thick-film heating element being welded to the mixing container, for example by means of laser welding. Furthermore, for better cleaning, the mixing container mayxe2x80x94as mentionedxe2x80x94also be provided with a removable lower part, it being possible here, for example, for the heating element to be molded-in in a lower part produced from plastic. With the resistance heating element formed as a thick-film heating element, it preferably comprises a stainless steel disk (substrate), which is provided on the underside with an insulating layer (dielectric). Applied on top of this, preferably by a screen printing process, are concentric resistance conductors consisting of ceramic pastes, again preferably in concentric circles. The distribution of the conductors over a large surface area produces a very uniform distribution on the heating element. The heat transfer from the heating element to the medium takes place by thermal conduction and is dependent on the thermal conductivity of the conductor, the dielectric and the stainless steel disk. The heat flow in the outer layer of the medium is determined by the size of the heating area, the heat transfer coefficient and the temperature difference between the medium and the stainless steel disk. Burning of critical foods is caused by excessive temperatures at the contact area of the heating. Critical temperatures may occur if the heat transfer coefficient deteriorates significantly. This is often the case with foods which contain starch, which become pasty from temperatures of 70xc2x0 C. and, owing to a sharp increase in viscosity, lead to a reduction in the heat transfer coefficient. If the heating power is not reduced, this is followed by a drastic rise in temperature at the heating surface on the inner side of the base of the mixing container. Apart from the main heat flow, an increased lateral heat flow in the direction of the disk area forms because of a lack of heat removal. To counteract this disadvantageous effect, a measuring sensor is provided, which while passing through a clearance in the dielectric rests against the outer side of the base. A temperature sensor of this type disposed in the direct vicinity of the conductor allows the increase in temperature to be detected and to be limited by switching off the heating. At the same time, a distance between the measuring sensor and the conductor of 2-8 mm is provided. This configuration has the effect of reducing the risk of burning by continuous monitoring of the temperature rise. In an exemplary configuration of the subject matter of the invention, an NTC element with a resistance of, for example, 10,000 ohms is provided for sensing the temperature, the measuring error caused by the existing contact resistance being negligible. In the case of a configuration of the heating element with a generally circular disk-shaped formation and a central recess which is adapted to the dome, the heat is transferred by thermal conduction to the dome of the mixing vessel, which dome secures the agitator. However, a construction in which the dome has a cylindrical region and a resistance heating element is formed in the cylindrical region of the dome is also conceivable. Said resistance heating element may take the form of a thick-film heating element or a tubular heating element, as described above. What is more, it is provided in an advantageous development of the subject matter of the invention that the heating disposed on the mixing container is electrically connected to the basic appliance via, for example, five contacts, that is a pair of contacts for operating the heating at line voltage, a pair of contacts for measuring the NTC resistance at, for example, 5 volts DC, and a protective-conductor terminal. The contacting undertakes not only the power supply to the heating but also the function of correct container detection. With a container inserted, the contact to the NTC (measuring sensor) is established, the resistance of which must be in a defined range. If this is not the case, switching on of the heating is prevented by the appliance software, to eliminate the possibility of a malfunction caused by incorrect resistance values. The contacting is disposed and constructed for example on the mixing vessel in such a way that cleaning of the mixing vessel in a conventional dishwasher is possible. Before removal of the mixing vessel, the voltage supply of the heating is electrically isolated by a switch, for example when a mixing vessel interlocking mechanism is actuated, and is only reactivated when the mixing vessel is locked.
The invention also relates to a food processor with a mixing vessel and a drive for an agitator in the mixing vessel, the mixing vessel being heatable in its lower region and having on the bottom a dome which has the securing mount for the agitator. Here it is proposed for the advantageous development of the subject matter of the invention that externally shielded microwave radiators are disposed outside the mixing vessel, associated with the bottom region. As a result of this configuration, contactless heating is provided for a food processor of the type in question, which is suitable for chopping up, grinding and pulverizing, beating, mixing, slow cooking, emulsifying and steaming. It proves to be particularly advantageous here that the microwave radiators comprise annular quartz-glass radiators which operate in the medium wave range and are disposed one above the other, so that a broad ring around the mixing vessel is created, with a defined distance from the outer surface of the mixing vessel. In this respect, a plurality of separately activatable quartz-glass radiators are preferably provided, that is again for example four annular quartz-glass radiators. The heating elements formed in this way can consequently be activated together, or else independently of one another, making optimized control possible. In addition, it is proposed that the mixing vessel is provided on the outer side of its lower region with a surface with active radiation absorption, which again for example is formed as a black coating. To provide heat protection from the interior of the appliance, it is provided that the microwave radiators are surrounded by a cooling duct. The latter is, in addition, preferably formed as an air duct with a through-flow.